1. Field of the Invention
The present invention relates to a vehicle driving-force control device that controls the driving force so as to properly maintain grip forces of wheels.
2. Description of the Related Art
In recent years, various types of vehicle driving-force control devices that suppress an excessive driving force to maintain grip forces of vehicle wheels have been developed and put to practical use.
For example, in a vehicle driving-force control device disclosed in Japanese Unexamined Patent Application Publication No. 2008-232080 (hereinafter referred to as Patent Document 1), friction-circle limit values of the wheels are calculated, requested resultant tire forces and resultant tire forces of the wheels are calculated, requested excessive tire forces and excessive tire forces of the wheels are calculated, an over-torque is calculated, and a control amount is calculated. This control amount is subjected to lower-limit correction using a first lower limit based on a road surface slope. If the first lower limit cannot be set because the road-surface slope cannot be obtained, lower-limit correction is performed using a second lower limit based on an accelerator opening. In this case, it is possible to inhibit not only an excessive driving force generated in the present, but also an excessive driving force to be presumably generated in the future and to properly maintain the grip forces of tires without interfering with uphill driving on a slope or the like.
However, in the driving-force control device disclosed in Patent Document 1, when the first lower limit is set on the basis of the road-surface slope, driving torque requested by the driver is not considered. Hence, even when the driver feels a necessity for a large driving torque, the lower limit is set at a small value based on the road-surface slope, and this may cause the driver to feel discomfort. In contrast, when the first lower limit based on the road-surface slope is not set, but the second lower limit is set on the basis of the accelerator opening (driving torque), the second lower limit is simply set in accordance with the accelerator opening. Hence, if the main transmission gear ratio is changed, the driving force changes even if the driving torque serving as the second lower limit is fixed. For this reason, the driving force becomes too small or too large, and this may also cause the driver to feel discomfort and may deteriorate driving feeling. Moreover, when the relationship between the accelerator opening and the driving torque for the second lower limit differs, a characteristic view showing the relationship between the accelerator opening and the driving torque needs to be set for each relationship, that is, a characteristic view needs to be set for each vehicle and for each relationship between the accelerator opening and the driving torque in the vehicle.